The classic Rankine cycle heat engine is illustrated schematically in FIG. 1. Most utility electric companies and many naval propulsion systems produce power from modified Rankine-cycle heat engines, usually using water as the working fluid. Compact versions of Rankine-cycle turbine engines have been built and demonstrated. Such systems are reliable, high-power-density power sources, and have been applied to specialized propulsion systems for undersea high-speed vehicles such as torpedoes.
In all practical engines with condensers, a fraction of the flow entering the condenser will be noncondensable gas, especially during the rapid start-up and early stages of operation of a compact engine. Most condensers include a subcooler before the high-pressure feed pump. The subcooler can be part of the condenser unit or a separate unit between the condenser and high-pressure feed pump. For simplicity, it is assumed that the condenser exit includes a subcooler before the condenser exit. In the case of compact once-through condensers, non-condensable gas is carried as bubbles along with the liquid to a condenser exit. Bubbles can interfere with production of well-controlled, steady power from the engine by causing the supply of liquid to a high pressure feed pump inlet to be unsteady or interrupted. Some provision needs to be made, therefore, to eliminate the noncondensable gases from the condensate flow before the working fluid enters the high-pressure feed pump.
In normal practice, a volume is placed in series between the condenser exit and the high-pressure pump inlet through which the working fluid flows. This volume is called a "hotwell" and utilizes gravity to form a free surface and bubble buoyancy to carry bubbles up to the free surface where they escape. The bubble-free inlet flow for the high-pressure pump is then taken from near the bottom of the hotwell. The thermodynamic state in the typical hotwell is near the boiling point of the liquid, and often special boost pumps are required to prevent cavitation in the high pressure feed pump. In addition to its function in bubble separation, the hotwell also serves as a reservoir for changes in working-fluid inventory during power-level and environmental changes and is often augmented with make-up fluid as necessary. The problem is that a gravity-driven liquid/gas separation process in the hotwell has proven to be inadequate for some propulsion applications.
The thermal efficiency of the Rankine engine is also affected by condensate handling and current hotwell design. The cycle efficiency is improved significantly by lowering condenser pressure. Lower condenser pressures reduce the backpressure at the turbine and lower the cycle heat-rejection temperature. Normal practice for lowering condenser pressure below ambient is to add a vacuum pump or ejector to extract accumulating noncondensable gas from the volume above the liquid surface in the hotwell, effectively lowering the pressure in the entire low-pressure portion of the working-fluid loop. The resulting low pressure in the hotwell expands the bubbles in the liquid, aggravating the compact-engine hotwell impact by entraining liquid in the vapor flow to the vacuum pump. Because of the local thermodynamic state, the vapor removed by the vacuum pump or ejector also contains a significant fraction of evaporated working fluid, continuously reducing the working-fluid inventory. Also, the lower pressure increases the risk of feed pump cavitation by lowering the net-positive-suction-head (NPSH) at the pump inlet, making the addition of a boost pump necessary.
In the case of a maneuverable vehicle using an engine with a hotwell, the lateral accelerations of the vehicle during a high-speed turn can shift the effective "g" vector to nearly horizontal axes. This causes the location and orientation of the free surface to be both variable and unpredictable in a maneuverable vehicle. In addition, some highly maneuverable vehicles require compact systems to meet severe volume constraints. As hotwell volumes are reduced and working fluid flowrates remain high, capillary and momentum forces in the high-rate flows overcome the ability of gravity forces to remove bubbles. There is, therefore, a minimum hotwell volume below which gravity-driven performance becomes marginal or unacceptable in even non-maneuvering conditions.
The object of the present invention is to improve the performance of compact, closed Rankine-cycle engines as well as other closed-cycle engines which pump condensate such as those used for propulsion of vehicles.